In various systems employed within the manufacturing, chemical processing and power generation industries and in other applications, equipment is coupled together by a communication network to form a distributed control system. Such communication allows for (1) transfer of data and (2) control of connected equipment. The network employs a broadcast bus to physically interconnect control and monitoring nodes associated with a physical process. A node, as used herein, refers to any device connected to the communication network for either supplying data (a Source) or receiving data (a Sink). A node may act as both source and sink. For example, one node may be a controller such as, for example, a General Electric Company Programmable Controller (PC) distributed under the brand `Series Six`, which PC may provide commands to operate equipment (source) and receive data (sink) indicative of the status of the equipment.
The communication network for factory control may include an overall broadband Local Area Network (LAN) supporting a number of smaller networks or "subnets". FIG. 1 illustrates an exemplary system in which a subnet carrier band LAN interconnects several devices forming a workcell. In such systems, it is important that each node be independent and perform its assigned task regardless of the other nodes. The communication network provides a data transfer function allowing each node to receive or generate data of interest. In some networks, data was sent to each node on the network, in sequence, with a separate message to each. In other words, sinks must "poll" sources; i.e., the sink node sends, in turn to all source nodes, a request message, receiving the data in separate response messages from each. More efficient networks use a broadcast technique in which each node sends all data onto the communication bus and all other nodes receive the data. The receiving nodes then evaluate and retain only data of interest to each.
A disadvantage of the above described broadcast technique is that each node must read each message on the network or subnet to identify data of interest to it. Since some data may be transmitted at high repetition rates and be part of a large group of data, time required for each node to identify and process selected data may affect system performance.
Another disadvantage is that, when nodes are added or deleted, existing nodes need to be changed to extend their knowledge of the system to these new nodes.